Recently, usage of aluminum alloy plates has been increasing for interior parts of automobiles and outer panels of home electric appliances. In either case, the aluminum alloy plates are required to have excellent surface quality in finished products. For example, when the plates are used for outer panels of home electric appliances, the plates are required not to have line patterns on the plate surfaces after anodic oxidation is performed. Various studies have been made in order to inhibit line patterns, and some methods have been suggested wherein chemical compositions, diameters of crystal grains in finished plates, sizes of deposits, distribution densities, and so on are controlled.
On the other hand, as the usage of aluminum alloy plates has become diversified, not only a plate surface, but also a cross-sectional surface of the plates is required to have good appearances. Specifically, the cross-sectional surface of the plates is required not to have line patterns after anodic oxidation is performed. In order to solve this problem, a method has been proposed so as to equalize the sizes of crystal grains in a cross-sectional surface of the plates. However, this method is not always effective in inhibiting the production of strip-like line patterns and therefore cannot be a reliable solution.